43. In this interrogative sentence the style is of severe reproach. It means: O Prophet (peace be upon him): Have these people become so bold and fearless that they do not feel any shame in accusing you of inventing a lie, and that too of a heinous sin against Allah? They calumniate you that you are forging the Quran yourself and then falsely attributing to Allah.
44. That is, such big lies are only uttered by those whose hearts have been sealed up. If Allah wills, He may include you also among them, but it is His mercy that He has kept you away from them. This answer contains a severe satire against the people who were so accusing the Prophet (peace be upon him). It means this: O Prophet, they think that you are a man like them. Just as they are in the habit of uttering any big lie only for the sake of a selfish motive, so they thought you also must have forged a lie for selfish ends. But it is Allah’s mercy that He has not sealed up your heart as He has sealed up theirs.
45. That is, it is Allah’s way that He does not grant stability to falsehood, and in the long run proves the truth to be true. Therefore, O Prophet (peace be upon him), you should go on doing your mission, without paying any attention to their false accusations. A time will come when this whole falsehood will vanish like dust, and the truth of that which you are presenting will become visible and manifest.
46. That is, He knows why you are being thus falsely accused and what are the actual motives that are working behind all this struggle that is being made to frustrate and defeat you.